1. Field of the Invention
This invention relates to seismic exploration, and, more particularly, to method and apparatus for simultaneously obtaining data from a multiplicity of common depth points.
2. Description of the Prior Art
In seismic exploration, seismic waves are commonly used to probe the earth's crust as a means of determining the type and location of subsurface formations. The earth's crust can be considered a transmission medium or filter of whose characteristics are to be determined by passing seismic waves through that medium. In the reflection seismic method, seismic waves or impulses are generated at a point at or near the earth's surface, and the compressional mode of these waves is reflected from subsurface accoustic impedance boundaries and detected by arrays of seismic detectors located on the earth's surface. The seismic detectors convert the received waves into electrical signals which are sensed and recorded in a form which permits analysis. Skilled interpreters can discern from such an analysis the shape and depth of subsurface reflection boundaries and the likelihood of finding an accumulation of minerals, such as oil and gas.
Various sources of seismic energy have been utilized in the art to impart the seismic waves into the earth's crust. These sources commonly employ an impulse type of seismic energy, which may comprise one or a series of pulses which are transmitted into the earth over a period of time. This transmission time is followed by a period of time for receiving the reflected waves. Common examples of such seismic energy sources are dynamite and a vibrating source of input signal.
Another type of source of seismic energy which has been utilized is the weight-drop system. This technique involves the dropping of a heavy weight from a truck at periodic intervals. However, the weight-drop seismic input system has not been utilized extensively due to the fact that efficient utilization of available recording time has not been made. This is due to the fact that in the past droptones were transmitted via radio to initiate each weight drop from each truck that was being used. This technique consumed valuable recording time with the radio transmission, and it is believed that up to 40% of recording could be lost. An example of such a system is disclosed in U.S. Pat. No. 3,902,161 to Kiowski, which discloses a radio transmission synchronization technique.
Another example of a synchronization technique is illustrated in U.S. Pat. No. 3,733,584 to Pelton. This technique involves bringing control units close enough to effect electrical transmission between them. Clocks in the control units are synchronized, and thereafter the control units are utilized to control the activation of seismic sources. However, it is believed that some sort of voice communication is required between the master and remote station to program times at which the sources are to be activated. The time required for this voice communication decreases the available recording time.
Accordingly, it is desirable to have a system for synchronizing and operating multiple energy source which makes efficient use of available recording unit. This useful result has been achieved by the method and apparatus of the present invention.